This invention relates to a method and apparatus for the controlled oxidation of materials, such as Al(Ga)As alloys.
In the optoelectronics industry, for example in the manufacture of LEDs and VCSELs (Vertical Cavity Surface Emitting Lasers), there is a need to effect the controlled conversion of Al(Ga)As to AlOx in order to fabricate current or optical apertures. The oxidation process for converting Al(Ga)As to AlOx is dependent on the kinetics of the oxidation reaction, supply of reactant (oxygen containing species) and removal of the by-products of the conversion reaction (e.g As). Unfortunately, this makes the Al(Ga)As-: greater than AlOx conversion process very sensitive to factors such as temperature of the sample, composition of the Al(Ga)As material, the thickness of the layer, surface treatment prior to oxidation, amount of moisture in the reactive atmosphere of the furnace, exposure to air prior to the oxidation process, and opening/closing mode of the furnace when the sample is loaded. While some of these factors, such as temperature, can be relatively easily, others are very difficult if not impossible to control accurately enough (e.g. exposure to air, loading mode) and are generally considered xe2x80x9cnoise factorsxe2x80x9d. The many investigations on various Al(Ga)As oxidation processes that have been carried out by different researchers indicate a large variability in the process, suggesting a large contribution by the noise factors. This makes it difficult to control the conversion process with sufficient precision, resulting in a degradation of the device performance and an inability to produce higher speed devices.
An object of the invention is to alleviate this disadvantage.
According to the present invention there is provided a method of carrying out the controlled oxidation of an oxidisable material, comprising the steps of placing the oxidisable material into a reactor, and causing a carrier gas to flow over said oxidisable material, said carrier gas containing an oxidising vapour at a controlled partial pressure.
The oxidising vapour is typically water vapour and the carrier gas, and inert gas, typically nitrogen, in which case the partial pressure of the water vapour can be controlled by controlling the temperature of a water bath, in turn to accurately control the oxidation process.
The oxidation process is typically carried out in a reactor tube at temperatures in the range of 400-500xc2x0 C. and pressures in the range of 50-100 mbar. These ranges are typical and not limiting. Precise control of the oxidation rate and of the final oxide thickness is achieved by controlling the partial pressure of the water vapor in the furnace tube.
The invention can be applied, for example, to the oxidation of Al(Ga)As in the manufacture of high speed VCSELs.
The process in accordance with the invention reduces the noise factors present when Al(Ga)As is converted AlOx. The oxidation process is designed so that rate depends mainly on one control factor, namely the partial pressure of the oxidising vapour or the time of exposure.
The time of exposure can be controlled in practice by removing the sample from the oven or just turning off the water vapour.
Using the method of the invention, it has surprisingly been found that the reaction parameters can be chosen so that the oxidation process is tolerant to variations in temperature and total pressure in the furnace, to variations in the composition and thickness of the Al-containing layers or to the preparation of the samples.
While primarily intended for Al-containing layers, such as Al(Ga)As, the invention can be applied to other materials that need to be oxidised in a highly controlled manner.
Typically, the reaction is carried out at a low pressure, for example, 50-100 mbar at temperatures in the range 400-500xc2x0 C. These parameters are purely exemplary and not limiting. Persons skilled in the art can determine the optimum parameters for any particular application by routine experiment.
The invention also provides an apparatus for carrying out the controlled oxidation of an oxidisable material, comprising a reactor for containing the oxidisable material, a supply of carrier gas for flowing over said oxidisable material, and a supply of oxidising vapour, and a mixing device for mixing said oxidising vapour with carrier gas at a controlled partial pressure.
The apparatus and process parameters presented here provide for the precisely controlled oxidation of layered semiconductor structures including at least one layer of Al containing alloy. As a particular case, an isotropic oxidation rate can achieved for an AlAs alloy without addition of Ga.